Polymeric electrochromics capable of a fast and reversible color change upon electrochemical oxidation and reduction have received considerable attention over the past two decades. A particular emphasis has been placed on incorporating the most stable of these electrochromic polymers (ECPs) in devices such as windows, mirrors (rear-view/side-view mirrors for cars) and displays, and other devices. Recently, synthetic conducting donor-acceptor (DA) polymers were demonstrated to have good performance as electrochromic polymers (ECPs).
The donor-acceptor approach, first reported for macromolecular systems by Having a et al., Synth. Met. 1993, 55, 299, allows the tuning of ultraviolet, visible, and near-infrared absorption bands in conjugated polymers (CPs) by the alternation of electron-rich donor (D) and electron-poor acceptor (A) segments. In these DA-polymers, the acceptors are easier to reduce, while the donors are easier to oxidize. The acceptor unit has a less negative reduction potential, as measured by cyclic voltammetry for the units hydrogen terminated monomeric species, than that for the equivalent hydrogen terminated donor unit. This approach has been used to tune the optical and electronic properties of CPs for applications such as field-effect transistors, light emitting diodes, and photovoltaics. To these ends, much effort has been directed to ECPs that are red, blue and green, the complimentary set of colors in the additive primary color space. Although ECPs of all of these colors have been prepared, they generally lack in optical contrast, switching speed, stability, processability or scalability of fabrication.
With respect to fabrication, the common synthetic approach to DA polymers has been through transition metal cross-coupling methods. In these methods, an electrophile, for example, an aryl halide, and a nucleophile, for example, an aryl-magnesium, are coupled, generally in the presence of a transition metal catalyst, for example, a nickel catalyst. In nearly all examples, as seems logically intuitive, the electron rich donor monomer is employed as the nucleophile as they are easily metallated and are highly reactive with electrophiles, whereas the acceptor units, for example, electron-deficient aryl halides, are used as the complementary electrophiles. Generally the weak carbon-halogen bonds are easily broken in the presence of the metal catalyst. Although DA polymers have resulted from such synthesis, those which have overcome all of the other barriers to implementation as ECPs in devices still lack for some colors, for example blue ECPs. Presently, only one DA polymer has been synthesized by cross coupling where the nucleophile is the acceptor, and the electrophile is the donor. (see Zhang et al., J. Am. Chem. Soc. 2007, 129, 3472 and Tsao et al., Adv. Mater., 2009, 21, 209) The alternating copolymer has benzothiadiazole (BTD) and 4,4-dihexadecylcyclopentadithiophene (CDT) repeating units and has been prepared by an “inverted” phase-transfer base-catalyzed Suzuki synthesis in a yield of 42 percent, which is not encouraging for commercial development of the polymer. This BTD-CDT DA polymer displays properties appropriate for organic field-effect transistors (OFETs) but its electrochromic properties have not been reported, and its neutral state absorption spectra is not attractive for development as an ECP.